High compression domes = snagged rings??

[ATS_Aaron]

I've got to talk about someone's bad luck other than mine....

Last Summer Scott installed some higher compression domes in his ~830cc stroker engine. He had been running about 155 psi and the ski was running great. The new domes put him at 200-205 psi compression.

He poured in some MS109 race fuel (and did not rejet) and we went out. At first the ski kept blowing the spark plug boots off. Once we figured out a way to hold then on with zip ties it blew up. I mean almost instantly.

The rear carburetor wasn't getting fuel. I have found in my many sad experiences that this means you have lost compression in the cylinder and no longer sending a pulse to the fuel pump.

So we pulled the head and sure enough the piston looked like it had bird footprints all over it. It had snagged a ring. It had no signs of detonation and the piston was not melted from being too lean. Scott found a piece of RTV sealant stuck to the bottom of his fuel pickup for that carburetor (left over from his internal fill conversion.)

So we chalked the failure up to a couple of things:

1. Too lean on the MS109 - it is oxygenated and you need more fuel.
2. Too lean on rear cylinder because of clogged/semi-clogged fuel pickup.
3. The heat from being too lean caused the piston and rings to get hotter than normal. This caused the ring to expand and to catch on the exhaust port. That broke the ring into pieces and sent it through the transfer ports where it ended up on top of the piston and gave us the bird footprints.

Continued....

 

[tor*p*do]

that is what sucks about duals in my opinion
one carb goes lean and you fry a cylinder quickly
single carb will run overall like cr@p
and give you notice and (hopefully) time to correct the problem

 

[ATS_Aaron]

In an effort to get back on the water last summer I gave Scott a used Yamaha stock 760 piston to replace his blown up one. He switched back to his lower compression domes and we rode the rest of the year and this year up until last week.

I forgot exactly why he pulled it apart last week, but it's a good thing he did (or so we thought). He found the rear cylinder/piston had lost it's Cir clip and the pin was starting to slide out and barely touch the cylinder wall. He caught that one just in time. He also found some issues with his low compression domes. So he decided to switch to the higher compression domes. No big deal because we knew why it blew up last time.....

So we get to the lake and Scott rides his ski for a few minutes..maybe 2. He's circling around letting it warm up and says the midrange/mid throttle feels great and very responsive. He gets it warmed up and goes full throttle. It feels GREAT..for about 2 seconds. Then it's back to one cylinder. He drags it to the shore and pulls the hood off and finds out he is not getting fuel to the front cylinder. I remind his that this is all de ja vue.

We get back to my house and pull the head. Yep same thing...except this time it is the front cylinder. It has snagged a ring just like last time.

1. NO MS109 this time - just 93 octane pump gas.
2. NO clogged fuel lines this time


Same ignition timing as the lower compression domes (28 degrees up to 5000 and -3 degrees per 1000 rpm after that). No signs of ignition detonation (although I will admit that I am used to looking at that on a car engine, not a 2 stroke).

No melted piston like from being too lean either. Just a snagged/broken ring on a cylinder that had ~50 hours on it already with the same rings.

The pistons/rings that have let go have been WSM. Not exactly top of the line products. The stock used Yamaha piston that I gave him last year is still in good shape.

So what is caused Scott's engine to blow up everytime he runs these high compression domes?

Aaron

Is it possible that we are running into a cheap piston problem and using a quality brand (Pro-X, Yamaha, Wiseco) piston would solve his issues?

 

[wsuwrhr]

Oxy fuels is really riding the razorblade in a two stroke.

Fall off on your setup or encounter a problem when running oxy fuels and this is the result...


I think 200-205 might be too high for pump gas.

Brian

I was worried when I saw the thread title....

 

[sflsurfrider]

the fact that theyre k-mart pistons that keep disintegrating could keep you running around in circles.

 

[cmeripper26]

OEM pistons if available! Heavier but reliable. Good luck with the fix.

 

[ryan]

wsm piston will not stand up to 220 psi. end of story!! you wil be luck if they run all year in a stock motor. what did you save in pistons? how many times has the motor ben apart? art/pro-x is the only way to go. un less you like changing pistons.

 

[ATS_Aaron]

I didn't save anything. I have Wiseco's in my ski ;-)

I'll let Scott know that the consensus seems to be better pistons.

Aaron

 

[ryan]

there just as bad

 

[BombThreat]

Really? Im wondering really because when I had my stroker built by Jetworks they only wanted to use Wiescos for reliability

 

[spork]

wsm piston will not stand up to 220 psi. end of story!! you wil be luck if they run all year in a stock motor. what did you save in pistons? how many times has the motor ben apart? art/pro-x is the only way to go. un less you like changing pistons.

Not sure I agree with that statement. I've had really good luck with wsm pistons. I'm running them very successfully in my 750 kaw and never had a failure. Granted i'm only running around 160psi.

 

[Idok]

we have had WSM pistons last for years in a STOCK or LTD boat....definatly not 200+ psi

same with SBT cranks...they work great, but not for ragged edge performance

 

[WFO Speedracer]

I will say this much about WSM pistons,they are fine for recreational use and that is what I use them for ,recreational engines,they do happen to use RK rings which are the exact same rings that PRO X uses,I have had zero problems out of either.They are cast pistons ,if you want to push the envelope in a two stroke engine and run race gas and super high compression you need to use forged pistons such as Wiseco's,they have greater resistance to detonation.I would be checking the champher on the exhaust ports before putting any piston back into those cylinders,I beleive there is where you will find your problem.

 

[ATS_Scott]

I will say this much about WSM pistons,they are fine for recreational use and that is what I use them for ,recreational engines,they do happen to use RK rings which are the exact same rings that PRO X uses,I have had zero problems out of either.They are cast pistons ,if you want to push the envelope in a two stroke engine and run race gas and super high compression you need to use forged pistons such as Wiseco's,they have greater resistance to detonation.I would be checking the champher on the exhaust ports before putting any piston back into those cylinders,I beleive there is where you will find your problem.

I went in and re-chamfered both cylinders after the first failure. Chamfers were smooth and more than adequate. I don't know why I thought the WSM's would survive the abuse.

Anyway thanks for the input, I have a set of Wiseco's on the way and some new pro-tec domes, so I should be running by this weekend.

 

[OCD Solutions]

A friend is having a very similar issue as this so I thought I would revive this thread. At first he thought he didn't have the chamfer correct so he played with it a bit more. After the second piston caught the ring he started looking closer and figured that he made the exhaust port too wide. I donated a 62T cylinder to the cause and he started fresh but was way more conservative on the exhaust port, well same thing, snagged a piston again.

He ran 24 gallons on the original cylinder with 38cc domes with no problems and only ran 6 gallons before snagging a ring with the 34cc domes and 205lbs compression. He started out with the 34cc domes in the second cylinder and ran over 30 gallons through it before snagging another ring.

If heat was the problem wouldn't you expect to see more sign of heat on the piston?

 

[keefer]

I am no expert, but I had the exact same failure some years ago when trying to use 200+ psi domes & 93 octane fuel with stock timing on a 84mm setup. To me it was not the ring that failed, it was the piston overheating on the exhaust side causing it to malform or crack thus causing the ring to hang. I had the carbs too lean and the 93 octane pump gas were my mistakes.

He would probably have been better off going back to the 109 octane fuel & rejetting richer. I am currently running 210psi on a 84.25 ProX pistons, stock stroke with 25 degrees timing on an enhancer. I am using 110 octane from a local pump. I have over 70 hours on this setup and everything still looks good.

 

[OCD Solutions]

Any idea what your ring gap was at? I can't help but wonder if opening up the gap would add any forgiveness?

 

[Don 79 TA]

well the ole-cliche goes....
live by the compression, die by the compression
you play.. you pay

but HOT dam.... when it works...... NOTHING better, VERY addictive

hate to see results like this... but i've always used sunnoco race gas, leaded, and so far no problems

i've lost motors due to using bad gauges... that sucked

 

[ryan]

bing go . ring gap is to tight!!! i have seen that kind of damage caused buy not checking ring gap. the ends touch and thats what ya get. the higher the compression the more it tends to happen. check it!!

 

[ATS_Scott]

Both of my failures snagged the ring on the bottom of the exhaust port, causing the top ring land, and piston crown to let go. It happened in a VERY short time. I think the rings heated up faster than the piston, gap closed, and the ring had nowhere to go but out. In my case i don;t think there was time to get the piston hot enough to deform. It looked perfectly fine except for the chunk missing.

I swapped in some wiseco's, new domes, and it's running great. Well it was until I pulled it out and installed the Lamey.

Your buddy's pistons don't look to have any heat damage either, no aluminum transfer to the bore, or melted edges. I think it's just too narrow a gap and crappy pistons, like me.